The field of the invention is that of assistance with aerial navigation and aerial safety and relates more particularly to assistance with the monitoring of the guidance of an aircraft along a trajectory within the framework of automatic flight control.
It may involve a descent trajectory or climb trajectory. In what follows, a descent trajectory will be taken as an exemplary trajectory, and an airplane will be taken as an exemplary aircraft. Such a trajectory 1 represented in FIG. 1 and called the vertical profile consists of a succession of rectilinear segments and is decomposed into two parts: a first part 10 during which the airplane is relatively free and which may therefore be optimized by adopting a preset speed and a thrust making it possible to minimize the consumption of fuel and a second part 11 for preparing the airplane for approach and for landing on a runway 12 during which the airplane must comply with certain parameters relating to altitude, speed and horizontal course by adopting a constrained speed and a constrained slope.
An airplane descent trajectory is computed in reverse starting from the configuration of the airplane on landing, then by backtracking and by establishing from point 2 to point 2 the corresponding speed preset and thrust preset until the point at which the descent begins or “TOD”, the acronym standing for “Top of Descent”.
Once this profile has been established by the flight management system (FMS) on the basis of constraints given by the air traffic controllers, the flight management system will give orders to the automatic pilot to join up with this profile and hold there, these orders being established as a function of control laws and of speed and thrust presets specific to the profile segment.
A distinction is made between the laws which determine the control of the elevators as a function of presets for speed (SPD), for vertical path (VPATH) or for vertical speed (VS) and the laws which determine the control of thrust and which are established as a function of presets for thrust (THR) or for speed (SPD). These laws are combined together and the resulting pairs of laws associated with guidance submodes, make it possible to establish the orders which will allow the aircraft to join up with (or stated otherwise capture) the profile segment or to hold there while complying with certain constraints.
From the above, the zone in which it is possible to capture the profile by applying a transition between the guidance submode which the airplane is in and the guidance submode adapted to the following of the profile segment to be captured is designated as the zone of capture about the profile. Outside of this zone a guidance submode making it possible to join up with this capture zone as quickly as possible is applied.
This capture zone 3 represented in FIG. 2 may be defined as a band of diameter D, fixed or depending only on the speed, centered on the profile 1.
However these definitions take no account of the differences in particular between a flight at high altitude and high ground speed (at the start of a descent for example) and a flight at low altitude and low ground speed (during for example the preparation for the approach procedure), as well as the corollaries such as flights at low altitude and high ground speed.
Thus for a flight at high altitude and/or at high speed, the capture zone is too restricted and the capture of the intended profile segment is too fast in particular for the comfort of the passengers; likewise for a flight at low altitude and/or at low speed, the capture zone is too wide and the capture of the intended profile segment is too lengthy in the sense that part of this capture time could have been devoted to another mode of flight and in particular to a faster capture with a more fuel-economical mode of flight such as for example the “airmass” mode.
Finally, the execution of the orders by the automatic pilot do not always make it possible to ensure the comfort of the passengers or to adopt a suitable mode of flight, during the capture of the profile.
It is known that, in order to ensure the comfort of the passengers, the movements of the aircraft must not lead to the vertical acceleration factor exceeding a certain threshold, for example equal to 0.1 g, g being the terrestrial acceleration (g=9.81 m/s2=32.1725 ft/s2). This factor must not be exceeded during capture either.
As a result, in order to ensure the comfort of the passengers, it is necessary to force the capture of the profile through trajectories subject to constrained load factors. The shape of the trajectory making it possible to preserve a constant vertical acceleration factor during capture is a parabola tangential to the profile.
To ensure the comfort of the passengers, it is also necessary to avoid big variations in thrust.
An important aim of the invention is therefore to choose a capture zone that is better adapted to the comfort of the passengers and to fastness of capture than in the prior art.